1. Field of the Disclosure
The present disclosure relates to novel polyvinyl chloride (PVC) and nanoparticle compositions and a method for producing them. In particular, the present disclosure relates to introduction of the nanoparticles at or above the fusion temperature of the PVC. The compositions are particularly used with cellulosic reinforcement fillers (e.g., wood flour) and preferably a coupling agent such as chitin or chitosan, or other relevant coupling agents for PVC and wood composites (e.g., aminosilanes).
2. Brief Description of Related Technology
Wood-Plastic Composites (WPCs) products have emerged as a new class of materials that can be used as alternative to solid pressure-treated wood in a variety of innovative applications, such as decking, docks, landscaping timbers, fencing, playground equipment, window and door frames, etc. (1,2).
Generally, WPC products have strength and stiffness properties that are somewhere between both materials (1,2). They are stiffer than neat plastics. Nevertheless, composites based on commodity plastics (e.g. polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinylchloride (PVC)) and wood-fibers do not offer mechanical performance similar to that of solid wood (1). For example, the flexural strength of WPCs made with commodity plastics are about two to three times lower than that of natural pine (softwood) or oak (hardwood), while the flexural modulus of WPC products is about one-half that of natural pine or oak (1). This lowered stiffness implies that, for the same load, a deck constructed with WPC products will bend more than a similar wood deck.
Improving upon the drawbacks of WPCs (e.g., lower flexural strength and modulus) could not only improve their acceptance in load-bearing structural applications, but also open new applications for these products, thus expanding their market share.
Several approaches with limited success have been proposed to substantially overcome the drawbacks of WPCs, including: (i) the use of higher wood fiber content (above 60 wt. %), (ii) the use of high performance plastics as a matrix, and (iii) modification of the matrix by incorporating nanoclay.
Increasing the amount of wood fiber reinforcements in WPCs can significantly enhance their stiffness due to the greater stiffness of the fibers. However, this approach also leads to a significant reduction in strength. In addition, the increased melt viscosity due to higher fiber loading makes processing more difficult (i.e., high pressure and torque, resulting in more energy being required), and the appearance of the final products suffers accordingly (e.g., poor surface quality, rough and tearing edges)(3).
The use of a high-performance polymer, such as poly(phenylene ether) (PPE), has also been proposed because of its greater strength and stiffness compared to commodity plastics (4,5). However, because of the high processing temperature of PPE (in the range of 280-320° C.), PPE can not be processed at lower temperatures (150-220° C.) needed to prevent the degradation of wood materials (2). A low molecular weight epoxy has been utilized to reduce the processing temperature of WPCs made with PPE (4). However, the epoxy acted as a plasticizer by softening the polymer, leading to lower strength and stiffness for the final products.
Recently, researchers have proposed to reinforce the matrix with nanoparticles and utilize these reinforced plastics (or nanoclay/plastics) as matrices for WPCs (6,7). Surprisingly, this approach has been unsuccessful. The results reported by Yeh et al. (6,7) has clearly demonstrated that both the flexural strength and modulus were considerably reduced as the amount of nanoclay (up to 20 wt. %) increases in WPCs made with 50% wood flour.
PVC has grown into one of the major thermoplastic materials, since it was first produced in the 1930's. There are a variety of PVC polymers available in the current commercial market. However, because of their inherent disadvantages, such as low thermal stability and brittleness, PVC products are subject to some limitations in certain applications. The common approach to overcome these drawbacks has been the utilization of a vast array of additives during the formulation of the resin.
During the past decade, nanocomposites based on nanoclay and polymers have been extensively studied as a newly developed polymer reinforcement technique (8). The use of nanoclay has been an attractive approach in the plastics industry to enhance the mechanical, thermal and barrier properties of the plastics even though the nanoclay amount is small (e.g., less than 10 wt. %). However, a homogeneous dispersion of nanoparticles (so-called exfoliation) in a polymeric matrix must be fully accomplished first to achieve the above-mentioned improvements.
The dispersion of nanoparticles into polymers is a challenge because of their strong tendency to agglomerate due to their high surface energy and large specific surface area. However, several approaches have been proposed to break down the agglomeration of nanoparticles during nanocomposite preparation. These approaches can be classified in two groups: (i) wet-based techniques, i.e., in-situ polymerization of a monomer with nanoparticles and solvent blending, and (ii) melt mixing of a polymer with nanoparticles. Each method has its advantages and limitations (9).
The synthesis of nanocomposites using wet-based techniques involves the dispersion of nanoparticles in water or organic solvents which must be properly disposed of to isolate the nanocomposites. Therefore, this approach is time consuming and not environmentally friendly.
In direct melt mixing approach (or dry mixing), however, the polymer, additives, and nanoclay are first dry-mixed and then melt-blended using conventional plastics processing equipment. Since this technique does not require solvent, it is more environmentally friendly. In addition, it is compatible with existing processing equipment, such as extruders, injection molders, mixing chambers (e.g., torque rheometers), etc., thus more effective in mass production of nanocomposites.
Since the extent of property improvement in nanocomposites is directly related to the degree of nanoclay dispersion, considerable efforts have been made to understand the formation of well-exfoliated nanocomposites via both melt processing and wet-based techniques. The properties of PVC-clay nanocomposites obtained via these two (2) methods have also been reported (8-11). Solution blending and in-situ polymerization method have been shown to be more efficient than melt mixing in improving mechanical properties at low clay content (e.g., 1-3 wt. %). However, at high nanoclay content, the effect of the preparation method in promoting the mechanical properties of nanocomposite is not significantly different from each other (8-10). Different trends have been reported by other authors where the mechanical and dynamic mechanical properties of an in situ PVC/CaCO3-nanocomposites exhibited much higher strength, modulus, toughness, and glass transition temperature than the nanocomposites prepared by direct dry-blending (11).
Most studies on PVC nanocomposites have centered on plasticized formulations (8,10). Relatively little in depth examination has been performed on rigid forms in particular. In addition, little attention has been directed toward the effect of dry-blending compounding method on the performance of PVC nanocomposites.
Several investigators have studied the performance of various polymers reinforced with carbon nanotubes (CNT). CNT exhibit superior thermal, electrical, and optical properties compared to diamond (12). They also have an extremely high elastic modulus, which is greater than 1 TPa (the elastic modulus of diamond is 1.2 TPa) and strengths which are 10-100 times higher than the strongest steel at a fraction of the weight (13). Particularly, the mechanical properties of polymer/CNT composites as function of carbon nanotube types, contents, and processing parameters have extensively been evaluated. Most of the polymer/CNT composites showed only a moderate or no strength/modulus enhancement, especially for polymer/CNT composites using untreated CNTs as reinforcement. The lack of improvement was mainly attributed to poor CNT dispersion within the matrix (12).
It is an object of the present disclosure to provide a novel method for forming composite compositions of PVC with nanosized fillers (e.g., nanoparticles). It is further an object to provide a method that is easy to perform and economical. These and other objects will become increasingly apparent by reference to the following description and the drawings.